After watching Titus Winters' "Live at Head" talk, where he mentions that StrCat() is one of people's favorite features, I decided to try and implement something similar to see if I could beat std::string::append (or operator+, which I figured uses append internally) in terms of runtime performance. My reasoning was that a strcat() function implemented as a variadic template would be able to determine the combined size of all its string-like arguments and make a single allocation to store the final result, rather than having to constantly reallocate in the case of operator+, which has no knowledge of the overarching context in which it's called.
However, when I compared my custom implementation against operator+ on quick-bench, I found that my strcat() implementation is about 4x slower than operator+ on recent versions of both clang and gcc, compiled with -std=c++17 -O3
. I've included the quick-bench code below for reference.
Does anyone know what could be causing the slowdown here?
#include <cstring>
#include <iostream>
#include <string>
// Get the size of string-like args
int getsize(const std::string& s) { return s.size(); }
int getsize(const char* s) { return strlen(s); }
template <typename S>
int strcat_size(const S& s) {
return getsize(s);
}
template <typename S, typename... Strings>
int strcat_size(const S& first, Strings... rest) {
if (sizeof...(Strings) == 0) {
return 0;
} else {
return getsize(first) + strcat_size(rest...);
}
}
// Populate a pre-allocated string with content from another string-like object
template <typename S>
void strcat_fill(std::string& res, const S& first) {
res += first;
}
template <typename S, typename... Strings>
void strcat_fill(std::string& res, const S& first, Strings... rest) {
res += first;
strcat_fill(res, rest...);
}
template <typename S, typename... Strings>
std::string strcat(const S& first, Strings... rest) {
int totalsize = strcat_size(first, rest...);
std::string res;
res.reserve(totalsize);
strcat_fill(res, first, rest...);
return res;
}
const char* s1 = "Hello World! ";
std::string s2 = "Here is a string to concatenate. ";
std::string s3 = "Here is a longer string to concatenate that avoids small string optimization";
const char* s4 = "How about some more strings? ";
std::string s5 = "And more strings? ";
std::string s6 = "And even more strings to use!";
static void strcat_bench(benchmark::State& state) {
// Code inside this loop is measured repeatedly
for (auto _ : state) {
std::string s = strcat(s1, s2, s3, s4, s5, s6);
benchmark::DoNotOptimize(s);
}
}
BENCHMARK(strcat_bench);
static void append_bench(benchmark::State& state) {
for (auto _ : state) {
std::string s = s1 + s2 + s3 + s4 + s5 + s6;
benchmark::DoNotOptimize(s);
}
}
BENCHMARK(append_bench);
That's because of passing arguments by value.
I changed the code to use fold-expressions instead (which looks a lot cleaner)
and got rid of unnecessary copies (
Strings... rest
should've been a reference).This solution beats
append
by approximately 30%.There seems to be no difference between passing by
const
references and doing perfect forwarding in this case. It makes sense, becausestd::string +=
won't move it's arguments even if they're rvalues.If you don't have access to the new fancy fold-expressions but still want the performance, use 'dummy array' trick instead (which seems to have exactly same performance in this case).